The present invention pertains to transmission of timing information and more particularly to a method of clock calibration.
Time-of-arrival (TOA) based positioning systems determine the location of a transmitter by measuring and processing the TOA of the transmitted signal at multiple locations around the transmitter. For the TOAs to be useful for position determination, they must all be based on the same time reference.
Existing multilateration systems rely on a means of synchronizing all the clocks in the system to a common time reference. This requires a means of simultaneously resetting all timers in the system to a common time.
If the timers are in close proximity and conditions permit, the timers may be reset by wire. But when the timers are distributed over a wide area, or where existing structures make it difficult to route wires to each timer, the synchronization signal must be sent via a radio link. The use of a radio link requires all timers to have an associated radio receiver to receive the synchronization signal, adding extra cost to each timer node. Another problem with the use of radio links to transmit the synchronization signal is susceptibility to multipath signal propagation delays. Yet another problem with a radio synchronization method in a complex indoor/outdoor environment is that multiple transmitters may be required to reach all timers in the system resulting in additional complexity and cost. It may be difficult to ensure that all transmitters transmit simultaneously.